2007 Chevy silverado (classic). Once hooked up to trailer, the brake lights do not work only turn signals on trailer.

Firat of all check and make sure you have a real good ground or otherwise you will lose current to another wire and this can short out and cause the lights to malfunction.

White: Ground
Brown: Running
Yellow: LT and Stop
Green: RT and Stop

This is the wiring code for the 4 wire trailers and
Wire a round seven-pin hitch, typically sold without a wiring harness, to the vehicle observing the following color codes: Run a white 10-gauge wire between ground and terminal one, run the blue 12-gauge wire between the electric brake controller and terminal two, the brown 14-gauge wire between the taillights and terminal three, the black 10-gauge wire between the permanently live 12-volt supply and terminal four, the yellow 14-gauge wire between the left turn/brake light and terminal five, the green 14-gauge wire between the right turn/brake light and terminal six and the purple 14-gauge wire between the backup light and terminal seven (in the center).